Aromatic dicarboxylic acids such as terephthalic acid and isophthalic acid are of great commercial importance and are widely used for the production of various polyester polymers such as fiber-forming and molding grade polyesters. Terephthalic acid (TPA) is one of the basic building blocks in the production of linear polyester resins used in the manufacture of polyester films, packaging materials and bottles. TPA used in the manufacture of such polyesters resins must meet certain minimum purity requirements. The purified condition of terephthalic acid refers primarily to the absence of significant concentrations of 4-carboxybenzaldehyde (4-CBA) and p-toluic acid that are present in significant quantities in the crude commercially-available grades of terephthalic acid. Both CBA and toluic acid are partial oxidation products formed in the manufacture of TPA by the catalytic oxidation of p-xylene. The purified form also refers to the absence of color bodies that impart a characteristic yellow hue to the crude material. The color bodies are aromatic compounds having the structures of benzils, fluorenones, and/or anthraquinones. 4-CBA and p-toluic acid are particularly detrimental to the polymerization process as they act as chain terminators during the condensation reaction between terephthalic acid and ethylene glycol in the production of poly(ethylene terephthalate) (PET).
Crude terephthalic acid may be purified by hydrogenation. In a typical hydrogenation process, the crude terephthalic acid is dissolved in water at elevated temperature and pressure and hydrogenated to convert the 4-CBA to p-toluic acid. The hydrogenation also converts the color bodies to colorless compounds. One constraint in developing a process for isolating the purified terephthalic acid from the hydrogenated liquor solution is the level of the 4-CBA and p-toluic acid acceptable in the final product. For example, purified terephthalic acid (PTA) typically contains on a weight basis less than 150 parts per million (ppmw) p-toluic acid. Isolation techniques to produce purified terephthalic acid use a wide variety of solid-liquid separation methods including crystallization, centrifugation, filtration, and combinations thereof.
Crude terephthalic acid obtained from the initial oxidation of a dialkylbenzene compound, normally p-xylene, typically contains a total concentration of 4-CBA and p-toluic acid of about 150 to 1100 ppmw based on the weight of the solids present. Crude terephthalic acid also contains lesser amounts, e.g., in the 20–200 ppmw range, of the characteristically yellow compounds. These compounds are colored aromatic compounds having the structures of benzil, fluorenone, and/or anthraquinone, which result from coupling side reactions occurring during the oxidation of p-xylene. It is necessary to purify the crude terephthalic acid when using it as a starting material for producing polyester fiber, which requires a purified terephthalic acid (PTA) as a starting material.
Such a purification process typically involves combining the crude terephthalic acid solid separated from the oxidation process with water to form a slurry thereof, which is heated to dissolve the crude terephthalic acid and impurities in the water and provide an aqueous solution. This solution is then passed to a reduction step in which the solution is contacted with hydrogen in the presence of a heterogeneous catalyst, usually palladium on a carbon support, at an elevated temperature such as 200 to 375° C. for the purification of TPA. The hydrogenation step converts the various color bodies present in the crude terephthalic acid to colorless products. The 4-CBA impurity is converted to p-toluic acid.
Subsequent separation and isolation of the terephthalic acid product can be accomplished via a wide variety of solid-liquid separation methods. A staged equilibrium crystallization approach is one separation method. With such an approach, evaporation is controlled against back pressure regulation in multiple crystallizer stages to control the rate at which the post-hydrogenation stream is crystallized. For terephthalic acid, it is believed that shock cooling of the post-hydrogenation stream to temperatures below 165° C. promotes the co-precipitation (co-crystallization) of impurities, particularly p-toluic acid, which contaminates the PTA product.
U.S. Pat. No. 3,931,305 discloses that the primary factor determining the impurity concentration in the terephthalic acid product is the lowest temperature to which the post-hydrogenation stream is flashed. The impurity concentration is less a function of the rate at which the post-hydrogenation stream is cooled. To this end, it is recommended that the majority of the terephthalic acid be crystallized at a temperature higher than about 160 to 182° C., which is the threshold temperature at which p-toluic acid co-crystallization becomes critical. When the post-hydrogenated stream of terephthalic acid has a concentration from 500 to 6,000 ppmw p-toluic acid, it is suggested to perform post-crystallization filtration at a temperature between about 121 and 149° C. to obtain a p-toluic acid concentration of 150 ppmw or less in the PTA product. Other isolation techniques use efficient filtration, washing, and drying methods within the temperature range of 100 to 205° C. to mitigate precipitation of p-toluic acid.